Carbon steels for machine structural use and low alloy steels for machine structural use have been used conventionally as the structural steels for the manufacture of machine structural components such as the components of cars, construction machines and the like. The machine structural components for cars such as bolts, rods, engine components and driving system components have so far been manufactured from these steel materials mainly through a hot forging and machining process. However, the recent trend is that the above hot forging and machining process is replaced with a cold forging process in view of advantages such as the improvement of productivity. In a cold forging process, cold forging work is usually applied to a hot rolled steel material after it is subjected to spheroidizing annealing (SA) and cold workability is secured. A problem here is that the cold forging causes work hardening of the steel material and its ductility is lowered, resulting in the occurrence of cracks and a shorter service life of metal dies. The occurrence of cracks during the cold forging work, or the insufficiency of steel ductility, often constitutes the main obstacle in the change from a hot forging process to a cold forging process, especially when heavy cold forging is required.
Meanwhile, in the spheroidizing annealing (SA), a steel material has to be heated to a high temperature and held there for a long time and, consequently, an apparatus for heat treatment such as a heating furnace is required and, in addition, energy is consumed for the heating and, for this reason, the spheroidizing annealing is responsible for a large proportion of the manufacturing cost. In view of the above, various technologies, such as those described below, have been proposed for the purposes of enhancing productivity, saving energy, etc.
For the purpose of reducing the time for the spheroidizing annealing, Japanese Unexamined Patent Publication No. S57-63638 proposes a method for obtaining a steel wire rod excellent in cold forging properties by cooling a hot-rolled steel material to 600° C., at a cooling rate of 4° C./sec. or higher, to form a quenched structure and then applying spheroidizing annealing to the steel material covered with scale in an inert gas atmosphere. For enabling quick spheroidizing, Japanese Unexamined Patent Publication No. S60-152627 discloses a method in which finish rolling conditions are specifically defined and a steel material is rapidly cooled after the rolling to obtain a structure where fine pearlite, bainite or martensite is mixed in finely dispersed pro-eutectoid ferrite. Japanese Unexamined Patent Publication No. S61-264158 proposes a method for lowering the steel hardness after spheroidizing annealing by improving the chemical composition of a steel, namely by obtaining a low carbon steel wherein the content of P is reduced to 0.005% or less and the expressions Mn/S≧1.7 and Al/N≧4.0 are satisfied. Japanese Unexamined Patent Publication No. S60-114517 proposes a method in which controlled rolling is applied for the purpose of eliminating a softening annealing process before cold working.
All these conventional technologies aim at improving or eliminating the spheroidizing annealing before the cold forging work and do not aim at improving the insufficient ductility of steel materials, which constitutes the main obstacle in the change from a hot forging process to a cold forging process in the manufacture of machine components requiring heavy working.